"The reason sparkling wine bottles are thicker than regular wine bottles is because they must withstand the pressure of the carbon-dioxide produced in the second fermentation which can reach 90 pounds per square inch."

6 atm

Wagner M., Philip. Grapes into Wine. 1976.

"The average champagne is fermented to a pressure of between 4 and 5 atmospheres or about 60 pounds pressure. The champagne called crémant (creamy) are less bumptious; their pressure is usually about 2 atmospheres."

"The pressure in a champagne bottle is typically between 70 and 90 pounds per square inch. That's two to three times the pressure in your car's tires, about the same as in a double-decker bus' tire."

5-6 atm

Champagne is a marvelous luxury to have to drink. It is used for many special
occasions and is known as the celebration wine. It is usually depicted
being used in anniversaries, wedding, and other special events as a bottle
of sparkling wine that has a cork that pops out of it like a bullet once
released. Then the special hoorays are heard and the foam starts spilling
out of the bottle like a fugitive escaping prison. This spectacle is one
of a kind however; this is not all fun and games. The speed of the cork
is very dangerous and can seriously hurt someone if they are hit with it.

The cork flies at such speeds when released because of the pressure built
up behind it. The second fermentation process of champagne, also known
as, en triage is where the pressure is built up. During the second fermentation process
of champagne, cane sugar and yeast is added to each bottle. The cane sugar
and yeast produces alcohol and carbon dioxide, which is the source of the
pressure in the champagne. This is also the reason why all champagnes have
extremely thick corks.

The pressure of champagne varies from different champagnes. The sources I
encountered ranged the pressure of champagne from 4 to 6 atmospheres or
in layman's terms, typically between 60 to 90 pounds per square inch. According
to USA Today, a pressure of this magnitude is only seen from double-decker
bus tires and champagne. This is the reason why you should never point
a champagne bottle at someone unless you really mean to hurt them.